1. Technical Field of the Invention
The present invention relates to a laser radar for three dimensional scanning, and in particular to a laser radar that three-dimensionally scan the space outside the laser radar using a laser beam.
2. Related Art
It is well known to use a laser beam to detect a distance to a target or direction of the target. A patent document JP-2789741-B, for example, discloses a device relating to such a technique. The device disclosed in this patent document includes a light isolator on the axis of a laser beam emitted from a laser beam generating means to transmit the laser beam and to reflect the light reflected from a target to a detecting means. Further, the device includes a concave mirror on the light axis of the laser beam transmitted through the light isolator. The concave mirror is adapted to rotate about the light axis of the laser beam to reflect the laser beam toward an external space. The concave mirror also reflects the light reflected from a target toward the light isolator to enable horizontal scanning covering an angle of 360°.
The technique disclosed in the patent document JP-2789741-B enables 360° horizontal scanning using the concave mirror and thus the detection range (scan range of a laser beam) is enlarged to the entire peripheral area of the device. However, this raises a problem that the detection range is limited to a plane. Specifically, since scanning is limited to a predetermined plane (scan plane), a laser beam reflected externally from the concave mirror which goes out of the scan plane is not able to conduct detection. Accordingly, a target which is present out of the scan plane cannot be detected. Even when a target is present in the scan plane, it is difficult to three-dimensionally detect the target.
To take measures against this problem, patent documents JP-2008-134163-A or JP-2009-098111-A discloses a technique that enables detection of a target in a three-dimensional space. For example, the patent document JP-2008-134163-A discloses a three-dimensional distance-measuring apparatus including a two-dimensional distance-measuring unit and a second rotating mechanism. The two-dimensional distance-measuring unit includes a rotating body that rotates about a given rotational axis. The second rotating mechanism rotates/drives the two-dimensional distance-measuring unit about a second axis obliquely intersecting the first axis. The second rotating mechanism includes a first bracket and a rotatable arm. The first bracket is pivotally supported about a third axis which is perpendicular to the second axis. The rotating arm is connected to a predetermined position on the first axis via a free joint mechanism. The rotating arm is rotated/driven by a driving mechanism to change a roll angle and a pitch angle of the first axis. Thus, the entire two-dimensional distance-measuring unit is pivoted to perform three-dimensional scanning.
However, the method of pivotally moving the two-dimensional distance-measuring unit as a whole together with its casing, as disclosed in JP-2008-134163-A, unavoidably increases the size of the operating mechanism (second rotating mechanism and the driving source (second motor)). This is quite disadvantageous from the aspect of reducing weight and size of the apparatus. Further, from the structural viewpoint of driving the two-dimensional distance-measuring unit in its entirety, mechanical or electrical load caused in the operating mechanism or the driving source is unavoidably large. Thus, considerably large torque, electrical power and the like are required in driving the unit, making it problematically difficult to perform scanning at high speed.
In particular, in the configuration disclosed in JP-2008-134163-A, a part (two-dimensional distance-measuring unit) driven by a three-dimensional-motion motor (second motor) is structurally much larger and heavier than a part (rotating body) driven by a horizontal-scan motor (first motor). Further, the motion provided by the second motor accompanies pivotal movement of the first bracket and the free joint mechanism. With this configuration, the motion of the second motor necessarily becomes slow, compared to the simple rotation of the light-weight rotating body on the side of the first motor. Accordingly, when high-speed scan is attempted by rotating the components of the first motor at high speed, the components of the second motor cannot follow the high-speed rotation. As a result, high-speed scan is encumbered.
On the other hand, the patent document JP-2009-098111-A discloses a laser radar having a configuration in which a laser beam from a laser diode is reflected to the side of a deflector by a pivoted mirror. In the laser radar, the pivotal movement of the pivoted mirror is controlled so that the direction of the laser beam incident on the deflector is changed. Thus, the radiation direction of the laser beam from the deflector is vertically changed.
As shown, for example, in JP-2009-098111-A, the configuration of displacing the pivoted mirror reduces the size and weight of the part (pivoted mirror) contributing to three-dimensional recognition, and thus reduces the mechanical and electrical load of the device, compared to the configuration disclosed in the patent document JP-2008-134163-A. However, in order to well perform three-dimensional recognition in a large rotation range using the configuration shown by JP-2009-098111-A, the pivoted mirror is required to be moved in a complicated manner. For example, in performing laser scan by pivotally moving the pivoted mirror as shown in JP-2009-098111-A, the driving of the pivoted mirror may be simplified and laser scan by the pivoted mirror may be based on a simple line scan (one-dimensional scan), so that high-speed driving is achieved.
However, this creates a phenomenon of not changing an incident angle (angle made between a laser beam emitted from the deflector and a horizontal plane) of the laser beam. This phenomenon is created when the deflector is at a rotational position where the deflector is oriented to a direction perpendicular to the direction in which the laser beam for line scanning is moved (perpendicular to the scan plane incident on the deflector). Thus, three-dimensional recognition is disabled in the vicinity of this rotational position. In order to eliminate such a problem, the pivoted mirror is required to be two-dimensionally moved in a complicated manner so that the laser beam is multidirectionally moved, instead of allowing the pivoted mirror to perform laser scan based on a simple line scan (one-dimensional scan). However, it is difficult to increase speed in such a complicated pivotal movement, and the complicated pivotal movement necessarily involves a complicated configuration and control method.